Advances in semiconductor integrated circuit (IC) technology in recent years have been remarkable, with exceptional trends in the miniaturization and increased integration of semiconductor elements in IC devices. So-called optical stepper apparatus and other projection-exposure apparatus that employ light for transferring the pattern from the mask to the substrate are in common use as microlithography apparatus for imprinting integrated circuit patterns on semiconductor wafers and other substrates. Such apparatus comprise a projection-optical system to effect transfer of the mask pattern to the substrate.
As used herein, a "transfer apparatus" encompasses any of various projection-exposure apparatus operable to perform projection-imprinting (transfer) of a pattern defined by a mask onto a surface of a sensitive substrate. Transfer apparatus include steppers and the like. Most transfer apparatus are "reducing" which means that the image of the pattern formed on the substrate is smaller than the corresponding pattern on the mask.
With continued advances in the miniaturization of circuit patterns, and in consideration of the resolution limitations of light, much recent research and development on microlithography has been directed to transfer apparatus that employ shorter-wavelength electromagnetic radiation (e.g., X-rays) or charged-particle beams ("CPBs", e.g., electron beams, ion beams) rather than visible or UV light.
Conventional masks used in CPB reducing-transfer apparatus (specifically an electron-beam reducing transfer apparatus) are shown in FIGS. 4 and 5(a)-5(b). Referring to FIG. 4, the mask 21 comprises a silicon mask substrate 22 that defines through-holes (voids) 23. The mask substrate 22 is sufficiently thick (e.g., 50 .mu.m) to absorb electrons irradiated onto the mask 21. Some of the electrons in the beam EB pass through the voids 23 and are focused by a pair of projection lenses 24a, 24b onto a "sensitive" surface (e.g., a surface coated with a resist) of a suitable substrate (e.g., a silicon wafer). Thus, a pattern collectively defined by the array of voids 23 is transferred to the sensitive substrate 25. Such masks are conventionally termed "stencil" masks.
Referring to FIGS. 5(a) and 5(b), a portion of another type of conventional mask 100 is shown. The mask pattern is defined in part by selectively applied regions of an electron-scattering material 30a formed on the surface of a mask membrane 20 usually made of silicon. The mask membrane 20 is sufficiently thin to allow electrons to easily pass therethrough. Masks in which a pattern is formed by defined regions of scattering material on a membrane lacking voids, as in FIGS. 5(a)-5(b), are termed "scattering transmission masks" or simply "scattering masks".
When electrons are irradiated onto the scattering mask 100, the degree of forward scattering of electrons is greater for the electrons EB2 that have passed through the electron-scattering material 30a and the mask membrane 20 than for the electrons EB1 that have passed through only the mask membrane 20. An aperture stop 7 (defining an aperture 7a) is situated in the vicinity of a crossover CO of the electron beam formed by refraction of the beam by the projection lens 5. Contrast in an image formed on the sensitive substrate 110 is achieved according to the degree of scattering of the electrons EB1, EB2.
Stencil masks typically have the following problems: (1) it is impossible to form an annulus-shaped feature with a continuous through-hole; and (2) most of the irradiating electrons are absorbed by the mask substrate 22 which heats the mask and causes the mask to experience substantial heat deformation and pattern distortion. To solve such a problem, attempts have been made to reduce the thickness of the mask substrate and/or to provide the stencil mask with an electron-scattering membrane to create a "scattering stencil" mask in which through-hole features are formed in the membrane.
With scattering transmission masks, since the scattering material 30a is supported by the membrane 20, annular shaped features can be formed, resulting in free-standing islands in the scattering material 30a, as indicated by feature A shown in FIG. 5(a).
Further with respect to a scattering transmission mask, since the scattering material 30a need not completely block irradiating electrons, the proportion of the electrons obstructed by such portions of the mask can be relatively small. Consequently, heat generation can be reduced compared to a stencil mask.
For use in making the mask membrane in scattering stencil masks and in scattering transmission masks, Si.sub.3 N.sub.4, Be, C (diamond), SiC, Al.sub.2 O.sub.3, Al, Si, SiO.sub.2, etc., have been evaluated. (Tungsten and gold have been evaluated as candidate materials for use as the scattering material in scattering transmission masks). Unfortunately, the oxide of Be is toxic, and light elements and light-element compounds such as Si.sub.3 N.sub.4, C (diamond), SiC, Al.sub.2 O.sub.3, Al, Si, SiO.sub.2, etc., exhibit low transmissivity to electrons due to the relatively short mean free path of electrons through such materials. Also, all of the conventional membrane materials are either monocrystalline or polycrystalline. Unless such materials are sufficiently thin, they are incapable of sufficiently suppressing absorption or scattering of the charged particles in the beam.
With a scattering transmission mask, the contrast of a projected pattern image is conventionally increased by increasing the scattering of the CPB in the scattering material 30a and decreasing the absorption or scattering of the CPB in the membrane 20. (With a scattering stencil mask, contrast is increased by decreasing the absorption of the CPB in the membrane.)
To such end, decreasing the thickness of the membrane 20 to approximately 10 nm has been studied in scattering transmission masks. Nevertheless, there remains a likelihood of the temperature of the membrane increasing excessively due to the radiant energy absorbed by the scattering material 30a, causing distortion of the mask (pattern distortion) and diminishing transfer precision. Also, if the membrane 20 is made too thin, its strength greatly decreases and the membrane becomes unable to support the scattering material 30a.
It is difficult for heat acquired by the scattering material 30a to escape. As a result, localized temperature increases can be extreme, especially in and around isolated islands of scattering material 30a (e.g., region A of in FIG. 5(a)).
Hence, in order to suppress absorption or scattering of a charged-particle beam in the membrane while preventing deterioration of transfer precision due to pattern distortion accompanying temperature increases in the mask membrane, the mask membrane should be very thin. In many instances, this requires that the mask include some additional structure providing thermal and mechanical support for the mask membrane. Consequently, contemporary masks for CPB microlithography are typically divided into multiple subfields each including a respective portion of the overall pattern defined by the mask. Each subfield is separated from its neighboring subfields by intervening boundary regions. The boundary regions do not define any portion of the pattern. Rather, each boundary region typically has an underlying structural feature. The structural features collectively form a comparatively rigid lattice substructure for the mask that thermally and structurally supports the entire mask. Such configurations are found in both scattering transmission masks and scattering stencil masks.
FIGS. 6(a)-6(b) show a conventional scattering transmission mask 100 for use in an electron-beam reducing transfer apparatus. The mask 100 comprises a regular array of multiple subfields 100a. Each subfield 100a contains a respective portion of the overall pattern defined by the mask 100 to be transferred to a sensitive substrate 110. The subfields 100a are separated from one another by boundary regions 100b that form a grid pattern across the mask 100. Referring to FIG. 5(b), supports Xa, Xb underlie respective boundary regions 100b of each subfield 100a and thus provide a structural grid for the entire mask. The mask 100 comprises a mask membrane 20 (FIG. 5(b)) that is transmissive to electrons. Applied to the upper surface of the mask membrane 20 within each subfield 100a are regions of an electron-scattering material 30a and spaces 20a therebetween that define, in each subfield 100a, the respective portion of the overall pattern defined by the mask.
When an electron beam is irradiated on a mask subfield 100a, electrons EB2 passing through the scattering material 30a and the membrane 20 experience a greater degree of forward scattering than electrons EB1 passing through the spaces 20a and the membrane 20.
A conventional electron-beam reducing transfer apparatus typically comprises a pair of projection lenses 5, 6 (FIG. 5(b)) for projecting the mask pattern onto a sensitive substrate 110. The apparatus also comprises a scatter aperture stop 7 defining an aperture 7a that transmits only those electrons passing through or near the crossover CO formed by the projection lens 5. As a result of the different degrees of electron scattering discussed above, and as shown in FIG. 5(b), most of the electrons EB2 scattered by passage through the electron-scattering material 30a and the membrane 20 are blocked by the scatter aperture stop 7. In contrast, most of the electrons EB1 passing through the spaces 20a and the membrane 20 pass through the aperture 7a.
An alternative scheme found in the prior art utilizes a scatter aperture stop 7 lacking a center aperture 7a and defining an annular aperture instead. With such a scheme, the resist is exposed in a differential manner that imprints a pattern whose features are defined by the scattering material 30a.
The mask subfields 100a are sequentially "transferred" to the sensitive substrate 110 subfield-by-subfield in each row and row-by-row. For example, as shown in FIG. 6(b), the subfields 100a are sequentially transferred by scanning the electron beam EB step-wise in the y-axis direction. After a row of subfields has been scanned, the mask 100 and substrate 110 are shifted in opposite directions along the x-axis, as shown by arrows Fm and Fw, respectively, to permit the next row of subfields to be transferred. (The subfield scanning sequence and the sequence by which the substrate 110 is exposed are indicated by arrows Am and Aw, respectively.) Such subfield-by-subfield transfer is termed "divided transfer".
During transfer, the mask and the substrate are moved in a coordinated manner. More specifically, the substrate 110 is moved in the -x direction at a velocity V.sub.W in synchrony with movement of the mask 100 in the x direction at a velocity V.sub.M. The reduction ratio imparted by the combination of the projection lens 5 and the objective lens 6 from the mask 110 to the substrate 110 is denoted by .beta.; the width of a mask subfield in the x direction is denoted by S.sub.x, and each boundary region has a width in the x direction denoted L.sub.x. The scan velocity V.sub.W of the substrate is expressed by: EQU V.sub.W =.beta..multidot.S.sub.x /(S.sub.x +L.sub.x)!.multidot.V.sub.M
Transfer of all the subfields of the mask defines one "die" on the substrate, and the substrate normally is exposed with multiple dies to form multiple devices.
When divided transfer is performed in the manner described above, the boundary regions 100b do not appear between adjacent "transfer subfields" 110b on the substrate. I.e., the transfer subfields are "stitched" together and made contiguous with each other on the substrate. To such end, a positional correction is made in the position of each transfer subfield by imparting a slight deflection of the electron beam EB after passing through each subfield 100a. The deflection is made in the y direction by an amount equivalent to the width Ly of the boundary region 100b. A similar deflection is also required (in the x direction) during each transition from one row of mask subfields to the next.
An exemplary mask 11 as would be used for X-ray projection exposure is shown in FIG. 7(a). The mask 11 comprises supports 13 on the lower surface of the mask membrane 12. (The mask membrane 12 is typically transmissive to X-rays.) The mask 11 also comprises X-ray absorbing regions 14 and "blind" regions 15 situated on the upper surface. The supports 13 provide thermal and mechanical support for the membrane 12, and serve to divide the membrane into multiple subfields 12a.
Transfer of the pattern defined by the mask 11 of FIG. 7(a) occurs by "divided transfer" as generally discussed above.
During manufacture of a mask of the types described above, when forming the pattern on the mask substrate, the pattern is not always formed as intended. Instances arise when the pattern is formed having certain defects.
If pattern transfer using a mask is used to expose only a portion of a die on a semiconductor chip, pattern defects would not be a large problem and there would be little need for pattern correction. Also, in instances in which a mask is used to expose an entire surface of a die, some pattern defects can often be tolerated due to, e.g., redundancy in the circuit. Nevertheless, only a few individual defects can usually be tolerated per chip. Also, many modern VLSI chips include regions in which defects absolutely cannot be tolerated. Thus, pattern defects have become a problem.